The invention relates to a fuel cell system that has an energy source backup.
A fuel cell is an electrochemical device that converts chemical energy produced by a reaction directly into electrical energy. For example, one type of fuel cell includes a proton exchange membrane (PEM), often called a polymer electrolyte membrane, that permits only protons to pass between an anode and a cathode of the fuel cell. At the anode, diatomic hydrogen (a fuel) is reacted to produce hydrogen protons that pass through the PEM. The electrons produced by this reaction travel through circuitry that is external to the fuel cell to form an electrical current. At the cathode, oxygen is reduced and reacts with the hydrogen protons to form water. The anodic and cathodic reactions are described by the following equations:
H2xe2x86x922H++2exe2x88x92 at the anode of the cell, and 
O2+4H++4exe2x88x92xe2x86x922H2O at the cathode of the cell. 
A typical fuel cell has a terminal voltage near one volt DC. For purposes of producing much larger voltages, several fuel cells may be assembled together to form an arrangement called a fuel cell stack, an arrangement in which the fuel cells are electrically coupled together in series to form a larger DC voltage (a voltage near 100 volts DC, for example) and to provide a larger amount of power.
The fuel cell stack may include flow plates (graphite composite or metal plates, as examples) that are stacked one on top of the other, and each plate may be associated with more than one fuel cell of the stack. The plates may include various surface flow channels and orifices to, as examples, route the reactants and products through the fuel cell stack. Several PEMs (each one being associated with a particular fuel cell) may be dispersed throughout the stack between the anodes and cathodes of the different fuel cells.
The fuel cell stack may be part of a fuel cell stack system that supplies electrical power to an electrical load. For example, for a residential fuel cell system, the electrical load may be established by the various power consuming devices of a house. To furnish AC power to the house, the fuel cell system typically converts the DC voltage that is provided by the fuel cell stack into AC voltages.
Because the power that is demanded by the devices of the house may vary, the fuel cell system may control the rate at which the above-described electrochemical reactions occur for purposes of regulating the efficiency of the fuel cell stack. In this manner, the fuel cell system may include a fuel processor to convert a hydrocarbon (natural gas or propane, as examples) into a reformate that contains the hydrogen gas. The rate at which the fuel processor produces the hydrogen gas flow needs to be large enough to satisfy the stoichiometry that is dictated by the above-described equation. A larger power demand from the house typically requires a larger flow rate and thus, requires a higher rate of hydrogen production by the fuel processor.
A conventional fuel processor may have a relatively slow transient response that causes any increase in its rate of hydrogen production to significantly lag the increased demand for power. As a result, when the power that is demanded by the house suddenly increases, the cell voltages of the fuel cell stack may significantly decrease due to the lack of a sufficient hydrogen gas flow until the rate of hydrogen production by the fuel processor increases to the appropriate level. Due to the delayed response of the fuel processor, it is possible that the fuel cell stack may be damaged. For example, if an electrical load is placed on a fuel cell that is not adequately supplied with hydrogen, the cell can go from generating power to itself being an electrical load as water at the anode is electrolyzed to supply the protons passing through the membrane. In such a case, a negative voltage arises across the cell (the cell xe2x80x9cgoes negativexe2x80x9d). This mode of operation in fuel cells has been known to irreparably damage the performance of such cells. Yet another problem arising from such a scenario is that the fuel cell is temporarily unable to meet the transient power demand.
Thus, there is a continuing need for an arrangement that addresses one or more of the problems that are stated above.
In an embodiment of the invention, a fuel cell includes a fuel cell subsystem, a battery and a circuit. The fuel cell subsystem is adapted to furnish power to a load. The circuit is adapted to connect the battery to the load when the fuel cell subsystem substantially delays in responding to a change in the power.
Advantages and other features of the invention will become apparent from the following description, from the drawing and from the claims.